How Much Concrete for a Slab: Sizes From 8×8 to 40×60
A 10×10 ft slab at 4 in needs 1.23 cubic yards of concrete, a 12×12 needs 1.78 yd³ and a 20×20 needs 4.94 yd³, before 10% extra.
Advanced options: waste, prices
NRMCA advises ordering 4 to 10 percent over the estimate.
Concrete needed (with 10% extra)
1.96 yd³
1.50 m³ · 48 ft³ before extra
Slab: 12 ft × 12 ft × 4 in. Over about 1 yd³: order ready-mix.
Premix bags
- 80 lb88
- 60 lb118
- 50 lb141
- 40 lb176
- Ready-mix cost
- $313
- Weight
- 7,200 lb
Full breakdown
| Volume before extra | 1.78 yd³ · 1.36 m³ |
|---|---|
| With 10% extra | 1.96 yd³ |
| Weight (150 lb/ft³) | 7,200 lb · 3.60 tons |
| Pallets of 40 lb (80 bags each) | 3 |
| Pallets of 60 lb (56 bags each) | 3 |
| Pallets of 80 lb (42 bags each) | 3 |
| Truckloads at 12 yd³ | 1 |
| Truckloads at 8 yd³ | 1 |
Bag counts round up, as Quikrete's own calculator does. Cost uses your prices; delivery and short-load fees are extra.
Key takeaways
- 10×10 = 1.23 yd³, 12×12 = 1.78, 20×20 = 4.94, 30×40 = 14.81 at 4 in.
- Add 10% for waste.
- Each extra inch adds 25% to a 4 in slab.
- 1 yd³ covers 81 sq ft at 4 in.
Concrete Needed by Slab Size
Concrete needed by slab size, from the same engine as the calculator, is listed below. Columns at 4, 5 and 6 in are before extra; the remaining columns use 4 in with 10% extra.
| Slab | Sq ft | yd³ at 4 in | at 5 in | at 6 in | 4 in + 10% | 80 lb bags | Trucks (12 yd³) | Concrete cost | Installed |
|---|---|---|---|---|---|---|---|---|---|
| 8×8 ft | 64 | 0.79 | 0.99 | 1.19 | 0.87 | 40 | 1 | $139–$169 | $416–$672 |
| 10×10 ft | 100 | 1.23 | 1.54 | 1.85 | 1.36 | 62 | 1 | $217–$265 | $650–$1,050 |
| 10×12 ft | 120 | 1.48 | 1.85 | 2.22 | 1.63 | 74 | 1 | $261–$318 | $780–$1,260 |
| 12×12 ft | 144 | 1.78 | 2.22 | 2.67 | 1.96 | 88 | 1 | $313–$381 | $936–$1,512 |
| 12×16 ft | 192 | 2.37 | 2.96 | 3.56 | 2.61 | 118 | 1 | $417–$508 | $1,248–$2,016 |
| 12×20 ft | 240 | 2.96 | 3.70 | 4.44 | 3.26 | 147 | 1 | $521–$636 | $1,560–$2,520 |
| 16×16 ft | 256 | 3.16 | 3.95 | 4.74 | 3.48 | 157 | 1 | $556–$678 | $1,664–$2,688 |
| 16×20 ft | 320 | 3.95 | 4.94 | 5.93 | 4.35 | 196 | 1 | $695–$847 | $2,080–$3,360 |
| 20×20 ft | 400 | 4.94 | 6.17 | 7.41 | 5.43 | 245 | 1 | $869–$1,059 | $2,600–$4,200 |
| 20×24 ft | 480 | 5.93 | 7.41 | 8.89 | 6.52 | 294 | 1 | $1,043–$1,271 | $3,120–$5,040 |
| 24×24 ft | 576 | 7.11 | 8.89 | 10.67 | 7.82 | 352 | 1 | $1,252–$1,525 | $3,744–$6,048 |
| 24×30 ft | 720 | 8.89 | 11.11 | 13.33 | 9.78 | 440 | 1 | $1,564–$1,907 | $4,680–$7,560 |
| 30×30 ft | 900 | 11.11 | 13.89 | 16.67 | 12.22 | 550 | 2 | $1,956–$2,383 | $5,850–$9,450 |
| 30×40 ft | 1200 | 14.81 | 18.52 | 22.22 | 16.30 | 734 | 2 | $2,607–$3,178 | $7,800–$12,600 |
| 40×60 ft | 2400 | 29.63 | 37.04 | 44.44 | 32.59 | 1467 | 3 | $5,215–$6,356 | $15,600–$25,200 |
How Much Concrete for a 10×10 Slab?
A 10×10 ft slab at 4 in needs 1.23 cubic yards of concrete, or 1.36 yd³ with 10% extra, which is 62 bags of 80 lb mix.
How Much Concrete for a 12×12 Slab?
A 12×12 ft slab at 4 in needs 1.78 cubic yards of concrete, or 1.96 yd³ with 10% extra, which is 88 bags of 80 lb mix.
How Much Concrete for a 20×20 Slab?
A 20×20 ft slab at 4 in needs 4.94 cubic yards of concrete, or 5.43 yd³ with 10% extra, which is one ready-mix truck; the concrete costs $869–$1,059.
How Much Concrete for a 20×20 Slab at 6 Inches?
A 20×20 ft slab at 6 in needs 7.41 cubic yards of concrete, or 8.15 yd³ with 10% extra.
How Much Concrete for a 30×40 Slab?
A 30×40 ft slab at 4 in needs 14.81 cubic yards of concrete, or 16.30 yd³ with 10% extra, which is 2 trucks at 12 yd³ (this calculator's own truck rule).
Concrete Slab Size Guides
Each common slab size has its own guide with bags, trucks, cost and uses:
- 10x10 concrete slab: small pads and sheds.
- 12x12 concrete slab: patios and shed bases.
- 12x16 concrete slab: larger sheds and patios.
- 20x20 concrete slab: two-car garages.
- 30x40 concrete slab: shops and pole barns.
- 40x60 concrete slab: large shops and barns.
Slab Sizes in Square Feet per Yard
One cubic yard covers 81 sq ft at 4 in, 64.8 sq ft at 5 in and 54 sq ft at 6 in, so divide your slab's square feet by these numbers for a quick yard count.
How to Use This Table for Other Sizes
- Find the closest size in the table for a quick check.
- Enter your exact length, width and thickness in the concrete slab calculator.
- Add parts for L-shaped slabs with the "Add another part" button.
Exact sizes and L-shaped slabs go in the concrete slab calculator.
Worked Examples: Reading the Table
20×20 shed base at 4 in. The table gives 4.94 yd³; with extra, 5.43 yd³, one truck, about $2,600–4,200 installed.
12×16 patio at 5 in. 12 × 16 × 0.417 = 80 ft³ = 2.96 yd³; with extra, 3.26 yd³.
10×12 shed pad at 4 in. 40 ft³ = 1.48 yd³; with extra, 1.63 yd³, or 74 bags of 80 lb.
How Much Concrete for a Shed Base?
A 10×12 ft shed base at 4 in needs 1.63 yd³ with extra, and a 12×16 ft base needs 2.61 yd³.
How Much Concrete for a 20×20 Slab per Square Foot?
A 4 in slab uses 0.0123 yd³ per square foot, so 400 sq ft takes 4.94 yd³ before extra.
How Much Concrete for a Slab FAQ
How much does it cost to pour a 20×20 slab?
A 20×20 ft slab at 4 in costs about $2,600–4,200 installed, or $869–1,059 for the concrete (Concrete Network national range).
Does a 4-inch concrete slab need rebar?
Not by code: the 2021 IRC treats slab reinforcement as optional; see the slab rebar calculator.
Is it cheaper to buy bags of concrete or mix your own?
For small jobs premix bags are simplest; mixing your own from cement, sand and gravel needs about 7 bags of 94 lb cement plus aggregate per yard, and above about 1 yd³ ready-mix is usually cheapest. See bags vs truck comparison.
How long does 4 inches of concrete take to harden?
A 4 in slab can take foot traffic after about 24 hours and a car after about 7 days, and should be kept moist for 3 to 7 days (Concrete Network; NRMCA). See concrete cure time.
Next step
Enter your exact size in the concrete slab calculator, then price it with the slab cost calculator calculator.
Official sources
Last reviewed 2026-11-05. Numbers come from our tested engine; see how we calculate and our sources.